20W-50 used in a 120 mercruiser engine?

[derf]

My 1972 mercruiser inboard calls for 30 weight oil, couldn't find any at the store so purchased 20wx50 thinking this should work as I'm running this boat in Lake Superior I think the Cold water keeps it running cooler so the viscosity shouldn't get to 50 weight,although I don't know what thermostat I have in there. But temp gauge dosen't get to high. and I'm running 40psi at 3k rpm's. any thoughts on this would be appreciated.
I was thinking of running 10W40 but I'm wondering if the viscosity would get to 30.

 

[OVERKILL]

Your understanding of how the motor oil viscosity system works in incorrect.

20w50 is a 50 weight at 100C
10w40 is a 40 weight at 100C

At any point BELOW 100C, they are HEAVIER than that.

The number BEFORE the W basically indicates the oil's resistance to thickening, it is not a viscosity number. For example, a 5w40 and 0w40 can both be an SAE grade of 40 at 100C, but the 0w40 will thicken less (usually... VI comes into play here, but that's not really necessary to discuss right now) as the temperature plummets.

The lower the number in front of the "w", the more resistant to thickening the oil is and usually this goes hand in hand with a lower MRV and lower pour point.

Each number listed in front of the W corresponds with a low temp MRV reference temperature.

for example:

0wXX is referenced at -40C
5wXX is referenced at -35C
10wXX is referenced at -30C
20wXX is referenced at -20C

There is a maximum thickness that the oil cannot be heavier than at this temperature, which varies accord to the grade, IE 20,30,40,50....etc. A 0w40 can be heavier at -40C than a 0w30, both have a maximum viscosity at -40C for their respective grade.

If your engine is running COLD, you want a LIGHTER oil in there, because your oil is never getting to the 100C temperature.

To better illustrate this, we'll use Pennzoil 20w50.

At -20C, it has an MRV of 25,600cP
at 40C it has a viscosity of 158.3cSt
at 100C it has a viscosity of 17.6cSt

SAE30 is between 9.3 and 12.5cSt at 100C.

17.6 is in the range for a 50-weight oil. 158.3 is obviously a LOT heavier than that. If your oil never gets above 40C (which I doubt but let us use that number as an extreme example) you are running an oil that is TWELVE TIMES HEAVIER than the limit of the SAE30 viscosity grade

 

[hemitom]

In the 120 mercruiser that i had i ran the merc 25w40 and shell rotella 15w40. Zero issues with either oil. I think you would be better served using the 15w40 over the 20w50. The diesel rated oil would be a better over all oil. Also make sure your engine is getting up to the correct operating temp, if not, you are creating a lot of fuel dilution and engine wear.

 

[derf]

Thanks a bunch,this really helped.

 

[Astro14]

Originally Posted By: derf
My 1972 mercruiser inboard calls for 30 weight oil, couldn't find any at the store so purchased 20wx50 thinking this should work as I'm running this boat in Lake Superior I think the Cold water keeps it running cooler so the viscosity shouldn't get to 50 weight,although I don't know what thermostat I have in there. But temp gauge dosen't get to high. and I'm running 40psi at 3k rpm's. any thoughts on this would be appreciated.
I was thinking of running 10W40 but I'm wondering if the viscosity would get to 30.


Calls for a straight 30, then use a 10W30 or a 5W30. Personally, I would choose a 10W30 like Rotella (heavy duty engine oil...diesel oil...stout and cheap)...but you wouldn't be far off in using a 15W40 like Rotella...

And re-read the viscosity explanation from Overk1ll...understanding in his post will make my recommendation clearer...

One other point: I am familiar with old oil specifications...the '32 in my signature calls for "medium cylinder oil" for fall conditions...and I follow the recommendation that I just gave you...

 

[OVERKILL]

FWIW, we have the same engine in our old Glasstron. We picked up some Delvac 1 5w40 and a Motorcraft filter to run in it.

I also changed the thermostat. They have an INSANELY cold thermostat, IIRC, it is 140F, which is ridiculous. My old Merc 888, which is a Ford 302 had a 180 in it. I tried to find a warmer (180) stat for it but Merc makes no such thing. It doesn't take a regular GM thermostat (I originally looked to see if I could go that route) or I would have just done that.

That being said, it seems to hover around the 150-160F mark flat-out, sometimes creeping up to around 170. At idle it is around 145F.

 

[Garak]

Originally Posted By: OVERK1LL
FWIW, we have the same engine in our old Glasstron. We picked up some Delvac 1 5w40 and a Motorcraft filter to run in it.


On the old Glastron my dad had, with the Mercruiser Chevy straight six, that was the first time I had every seen an oil change from the dipstick tube. Oh, and my dad stuck to his tried and true 10w-30.

 

[OVERKILL]

Originally Posted By: Garak
Originally Posted By: OVERK1LL
FWIW, we have the same engine in our old Glasstron. We picked up some Delvac 1 5w40 and a Motorcraft filter to run in it.


On the old Glastron my dad had, with the Mercruiser Chevy straight six, that was the first time I had every seen an oil change from the dipstick tube. Oh, and my dad stuck to his tried and true 10w-30.

Hehehe, we'll probably try the new full SAPS AMSOIL Euro in it next year, though the SS 0w30 I've been running in the Expedition would probably be fine too.

 

[Fordiesel69]

For this marine engine a nice 5w-40 or 15w-40 diesel oil is well suitable. You should not use 5w-30 or 10w-30 for marine because of the high load / high temp / fuel dillution.

I have the same engine and it runs great on 15w-40 delvac. Does not burn a drop.

 

[friendly_jacek]

Originally Posted By: Astro14


Calls for a straight 30, then use a 10W30 or a 5W30.


Bad advice. The poster didn't realize we were talking boats here (posted in the wrong forum).